1. Field of the Invention
The present invention relates to a multicast network unit (a multicast network apparatus), a multicast network system, and a multicast method, which are suitable for systems that deliver voice, video, and data services.
2. Description of the Related Art
In recent years, with the spread of the Internet and speedup of communication lines, delivery service is being widely performed which multicasts IP packets containing content data (such as voice, still images, motion images, etc.) across a network to a great number of delivery destination servers (access servers) that are accessed by users. In the multicast communication, one delivery source server assigns a multicast address having a group identification function to each group (which has a plurality of servers as members).
Multicast protocols will be described.
Typical multicast protocols are several kinds of multicast routing protocols such as DVMRP (Distance Vector Multicast Routing Protocol), PIM (Protocol Independent Multicast)-DM, PIM-SM (Sparse Mode), etc. Among the protocols, PIM-SM suitable for large-scale networks is often employed. Particularly, the case of constructing one multicast network (multicast domain) by the bootstrap routers (BSRs) in PIMv2 will be described with reference to FIGS. 17A and 17B.
(i) Determination of a Rendezvous Point (RP)
FIGS. 17A and 17B show how routing is carried out by PIM-SM. If a delivery destination server 200 shown in FIG. 17A transmits a data delivery request to a router 201a, the router 201a transmits the data request to a rendezvous point (RP) 202 corresponding to the multicast address. A source server 203 delivers data to the RP 202. This rendezvous point 202 represents a delivery point, and a plurality of rendezvous points are provided for each content data such as an image 1, an image 2, etc.
(ii) Bootstrap Router and Rendezvous Point 202
A bootstrap router receives packets from a plurality of rendezvous points 202, within its managing ability and learns delivery information held in each rendezvous point 202. The bootstrap router also sends the learned delivery information and the determined RP information to other routers.
Delivery systems employing multicast communication will hereinafter be described.
(i) Delivery System Having a Single Domain
(Network Domain or Multicast Network Domain)
FIG. 18 shows the construction of a delivery system employing a single domain. The delivery system (multicast network system) shown in the figure is, for example, a system that delivers image data. The delivery system has, for instance, seven networks 11, 22, 23, and 31 to 34. Each of the networks 11 to 34 has one or more layer-3 switches (L3-SWs) 90a to 90e that transfer packets. In addition, encoders 21 for delivering image data and decoders (dedicated devices, personal computers, etc.) are connected to each network through routers.
In these networks 11, 22, 23, and 31 to 34, a multicast protocol operates as one multicast domain, and rendezvous points corresponding to multicast addresses are managed by one bootstrap router.
The layer-3 switches 90a to 90e are routers, which retransmit a transmitted packet at high speed by referring to both the IP address (network layer) of the other server contained in the packet and the data (connecting tree) in which a multicast address and a route correspond to each other. Each of the layer-3 switches 90a to 90e is connected to the personal computer (PC) or workstation (WS) of a user through a layer-2 switch (not shown) provided in each communication subnetwork. The layer-2 switch is a bridge, which retransmits a transmitted packet by referring to the MAC address (data link layer) of the other server contained in the packet.
In this way, a route that becomes a rendezvous point is determined for each image source, and if each layer-3 switch receives a data delivery request from a delivery request source, it transmits the data delivery request for image data to the rendezvous point. Similarly, when another L-3 switch receives the delivery request for that image source from another delivery request source, it transmits the data delivery request to the rendezvous point. If there is a route along which the image has been delivered, the layer-3 switch uses a tree construction containing that route. Therefore, since multicast packets are transferred onto only the required route, a reduction in traffic can be decreased.
(ii) Delivery System Having a Plurality of Domains
FIG. 19 shows the construction of a delivery system to which a plurality of domains are connected. In the delivery system shown in the figure, three networks 11, 22, and 23 differing in domain are interconnected through an external network 101, etc. Between the networks 11, 22, and 23, peer-to-peer communication is performed.
Conventional Multicast techniques that have been proposed are disclosed in the following publications.
A gateway device disclosed in e.g. Patent Document 1 relays multicast packets only during the time the packets are being delivered. This allows only the required multicast packets to pass through this device without having influence on other applications or servers, so multicast packets are safely relayed.
In a multicast method disclosed in e.g. Patent Document 2, a public data packet uses a public multicast address and is multicast through a public data packet network. Therefore, network resources (bandwidth capacity) are efficiently used.
In a packet conversion method disclosed in e.g. Patent Document 3, packet conversion devices, provided at the inlet and outlet of a network that supports no multicast communication, convert a multicast packet to a unicast packet to perform communication. This makes multicast communication possible between two kinds of local area networks constituting a virtual public network.
In a multicast method disclosed in e.g. Patent Document 4, when a satellite line is normal, multicast packets are prevented from being sent and received to and from the Internet. Also, when the satellite line is not normal, multicast packets are allowed to be sent and received to and from the Internet. Therefore, a large-scale multicast process becomes possible by employing an IGMP (Internet Group Management Protocol) proxy server.
In a multicast method disclosed in e.g. Patent Document 5, a router proxy server judges if multicast data is delivered from an upper network to a lower network, in dependence on a predetermined criterion, and a network controller delivers multicast data in dependence on the judgment. Therefore, the utilization efficiency of networks is enhanced.
A network unit disclosed in e.g. Patent Document 6 is provided with a conversion data table, in which a multicast address, a unicast address of a terminal, and a port number of a network unit corresponding to a multicast frame and connected with the terminal correspond to one another. Therefore, even when a network unit not corresponding to a multicast frame is used, data can be delivered only to the terminals of a specific group. As a result, the bandwidth utilization efficiency of a network is enhanced, reliability can be improved.
In a multicast communication method disclosed in e.g. Patent Document 7, a sending host sends a message having a group identifier to receiving hosts that have requested that message, at the same time. This permits a message to be transmitted to many receiving hosts at the same time. In addition, a message can be simultaneously transmitted on a very large scale that cannot be realized by one multicast server.
In a multicast gateway communication method disclosed in e.g. Patent Document 8, a multicast gateway receives a message from a sending host, copies that message, and individually sends it to a plurality of receiving hosts as an IP unicast datagram. And the multicast gateway converts the received message, which contains an IP multicast address as a destination, to a message having a group identifier. The converted message is individually sent as an IP unicast datagram to a plurality of receiving hosts that have requested that message.
Therefore, a message can be transmitted simultaneously to receiving hosts that have not supported IP multicast communication.
A communications device disclosed in e.g. Patent Document 9 transfers multicast data between networks by employing an IP multicast address given to a receiving terminal, etc. This makes a network address conversion process unnecessary and also makes high-speed transfer possible at low cost. In addition, a considerable simplification in device construction and a reduction in cost become possible.
In a multicast transfer system disclosed in e.g. Patent Document 10, a multicast node arranged in an ATM network makes a multicast transfer tree where edge nodes are destination nodes, for each user connected to each edge node and dynamically changes that multicast transfer tree in dependence on a control message. Therefore, this system is capable of reducing the load of each relay node and easily coping with node change.
Patent Document 1                Japanese Laid-Open Patent Publication No. 2000-32049        
Patent Document 2                Japanese Laid-Open Patent Publication No. 2001-77859        
Patent Document 3                Japanese Laid-Open Patent Publication No. 2001-230774        
Patent Document 4                Japanese Laid-Open Patent Publication No. 2002-9848        
Patent Document 5                Japanese Laid-Open Patent Publication No. 2002-152197        
Patent Document 6                Japanese Laid-Open Patent Publication No. 2003-298602        
Patent Document 7                Japanese Laid-Open Patent Publication No. HEI 10-63598        
Patent Document 8                Japanese Laid-Open Patent Publication No. HEI 10-242962        
Patent Document 9                Japanese Laid-Open Patent Publication No. HEI 10-308758        
Patent Document 10                Japanese Laid-Open Patent Publication No. HEI 11-298498        
However, if the scale of a network employing a multicast protocol is enlarged, a bootstrap router has to process a load that exceeds its managing ability, because the device has a limit to its managing ability. In addition, the number of rendezvous points representing a delivery point is increased. Because of this, bootstrap routers will exceed various kinds of processing abilities. Furthermore, the size of the management table for bootstrap routers is increased, so that traffic congestion occurs.
In multicast networks employing peer-to-peer connection, domains between different networks are connected with one another. On the other hand, the multicast domains 22, 23 and external network 102 are able to refer to the address information (such as multicast addresses, delivery-point (rendezvous-point) information, and multicast source information) within the domains, so there is a problem that security cannot be enhanced.